1. Field of the Invention
The present invention relates to a thermally developable light-sensitive material, and more particularly, to a thermally developable light-sensitive material in which thermal fog (i.e., fog formed during thermal development) is minimized and freshness retention property (the capability of retaining, even after storage, the photographic characteristics initially possessed by the thermally developable light-sensitive material immediately after production, especially as to sensitivity) is improved without increasing both light discoloration (i.e., coloration occurring at the background of the light-sensitive material after development upon storage exposed to light) and dark discoloration (i.e., coloration due to heat and humidity in the background of the developed light-sensitive material on storage in the dark).
2. Description of the Prior Art
Silver halide photography has been employed most widely for a long time, because silver halide photography has superior photographic properties such as sensitivity and gradation to those of electrophotography or diazo photography.
Generally, a silver halide light-sensitive material for use in silver halide photography is imagewise exposed to light followed by development with a developing solution and subsequently is subjected to several wet processings such as stopping, fixing, washing or stabilizing so that the thus-obtained image does not rapidly discolor nor fade under normal room illumination and so that the undeveloped white portion (background) is not blackened.
Accordingly, many problems arise with silver halide photography, for example, the processings take a long time and are laborsome, handling of the chemicals is hazardous, staining of clothing and the processing room occur and, still further, pollution problems arise when the used processing solution is discharged.
As a result, a high-sensitivity silver halide photographic light-sensitive material suitable for dry processing which can be subjected to processing without the necessity for wet processing and which provides a stable image and further has minimized discoloration at the background under normal room illumination following the dry processing has been desired.
At the present, the most successful light-sensitive material, which can be used to form a photographic image thereon by dry processing, is a thermally developable light-sensitive material containing a composition comprising, as essential components, a silver salt of an organic acid, a small amount of a photocatalyst such as silver halide and a reducing agent, as described in U.S. Pat. Nos. 3,152,904 and 3,457,075.
In this type of light-sensitive system, the photocatalyst such as silver halide remaining in the light-sensitive material after development is allowed to discolor without being stabilized against light, yet substantially the same result as that attained by stabilization is obtained. This is because the photocatalyst such as a silver halide is used in a minor proportion whereas a white or lightly colored organic silver salt is used in a major proportion and is sufficiently stable that darkening upon exposure to light hardly occurs. Accordingly, even if the photocatalyst such as a silver halide discolors due to light, since it is present in a minor amount, the background has a white or lightly colored appearance overall and, therefore, this slight discoloration hardly causes any difficulty.
This light-sensitive material is stable at room temperature. However, when the light-sensitive material is imagewise exposed followed by heating generally at a temperature of about 80.degree. C. or higher, preferably at 100.degree. C. or higher, the organic silver salt oxidizing agent and the reducing agent which are incorporated in the light-sensitive layer thereof undergo an oxidation-reduction reaction due to the catalytic activity of the photocatalyst such as exposed silver halide in proximity to the organic silver salt. As a result, the exposed portion of the light-sensitive layer rapidly blackens and a contrast with the unexposed portion (background) thereof occurs, which results in image formation.
As described above, the present invention is directed to an improvement in thermally developable light-sensitive materials and, particularly, has been accomplished by adding thereto, as a stabilizing agent, component (d) which gives rise to the effects as described hereinafter.
Incorporation of organic carboxylic acids such as phthalic acids, benzoic acids, and long chain-carboxylic acids, or sulfonic acids into a thermally developable light-sensitive material is described for example, in Japanese Patent Application (OPI) Nos. 97523/1973, 89720/1973, 10039/1974, Research Disclosure, p. 20 (11723), January, 1974, and Japanese Patent Application (OPI) No. 125016/1974. (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".)
However, it was found that these acids do not provide any advantages in a thermally developable light-sensitive material which employs silver behenate as the primary organic silver salt component. For example, the use of phthalic acids such as phthalic acid and tetrachlorophthalic acid increases the light discoloration and increases the dark discoloration. In addition, it was found that benzoic acids such as benzoic acid, p-hydroxybenzoic acid, salicylic acid, p-t-butylbenzoic acid, p-methylbenzoic acid and anisic acid hardly have any activity as a thermal fog-preventing agent. Further, it became clear that tetrabromobenzoic acid and tetrachlorobenzoic acid also have such a weak thermal fog-preventing activity that these acids decrease the D.sub.max. Still further, it became clear that p-toluene sulfonic acids and benzene sulfonic acids decrease the D.sub.max of the image and, in addition, increase the light discoloration and the dark discoloration.
In order to solve the numerous problems in the prior art as described above, various investigations have been made energetically and the present invention has been accomplished.